Multi-level digital signals are conventionally transmitted through a variety of media, for example, hard-wire lines, fiber optic cable, and radio communication links. These signals are transmitted at a known rate referred to as the data rate or clock signal frequency. During the course of the transmission, these signals often become degraded by receiver and transmitter component variations, noise, and other types of interference, causing fluctuations in the received signal. These fluctuations cause inaccurate demodulation at the receiver such that symbol errors occur, and, since symbols are comprised of bits, bit errors result, causing distortion in the reconstructed data.
The resultant fluctuations in the received signal are more destructive in a multi-level digital signal than in a binary signal because the levels being distinguished between are much closer in a multi-level signal because of limited signal bandwidth in FM signals and power limitations in linear systems. The digital discriminator output signal is described as an eye pattern, because of the shape the discriminator output takes on an oscilloscope display, and the distance between expected voltage levels in a digital signal is referred to as an eye opening. For example, an error-free binary signal may have an 6.0 V eye opening, and an error-free 4-level signal may have an opening of 2.0 V. When fluctuations or noise enter by or in the receiver, the opening of the eye tends to close, as would a human eye in a sandstorm. The smaller the eye opening, the more difficult it is to recover the data accurately. Simply fixing the decision threshold at the expected halfway point between adjacent symbol levels will not suffice further away from the center bias of the signal, as distortion is more likely to occur at those levels than near the center bias.
Accordingly, there is a need for a method of accurate symbol recovery for multi-level signals, which has symbol thresholds that can be adjusted to various forms of signal degradation.